1. Field of the Invention
This invention relates to a plastic optical fiber, an optical fiber cable and an optical fiber cable with a plug which have a reduced light-transmission loss; processes for manufacturing a methyl methacrylate polymer with improved optical properties; and processes for manufacturing a plastic optical fiber.
2. Description of the Prior Art
A methyl methacrylate polymer may be prepared by a polymerization process such as suspension polymerization, solution polymerization and bulk polymerization; bulk polymerization and solution polymerization are advantageous for reducing light-scattering matters such as dusts and impurities in a polymer. In particular, bulk polymerization is most advantageous because solution polymerization requires removing not only unreacted monomers but also a solvent. JP-B 5-32722 has disclosed a process for manufacturing a plastic optical fiber comprising the steps of preparing a methyl methacrylate polymer with improved optical properties and containing a reduced amount of light-scattering matters such as dusts and impurities and light-absorbing matters such as peroxides and oligomers, and then forming a plastic optical fiber using the polymer as a core component. In the process, the polymer is prepared using a radical polymerization initiator represented by formula (I) (hereinafter, referred to as an xe2x80x9cinitiator Ixe2x80x9d) such that there is a relationship between an initiator concentration and a polymerization temperature satisfying a particular condition. 
However, since the initiator I is used in the process of JP-B 5-32722, a polymer obtained has a terminal C5H11 moiety different from a methyl methacrylate unit, which causes an uneven molecular structure and deteriorated optical properties in the polymer. When using this polymer for preparing an optical fiber, the optical fiber exhibits inadequate transmission performance. Thus, there has not been provided a process for manufacturing a methyl methacrylate polymer with adequately improved optical properties, or for manufacturing a plastic optical fiber exhibiting satisfactory transmission performance.
An objective of this invention is to provide processes for manufacturing a methyl methacrylate polymer having adequately improved optical properties and for manufacturing a plastic optical fiber exhibiting satisfactory transmission performance.
This invention provides a process for manufacturing a methacrylate (co)polymer comprising conducting polymerization while feeding a monomer (mixture) containing at least 90 wt % in total of at least one methacrylate monomer and a radical polymerization initiator represented by formula (II) into a reactor, where an initiator concentration and a polymerization temperature satisfy a relationship represented by equations (1) to (4) and the polymerization temperature is not less than 110xc2x0 C. and not more than 160xc2x0 C.;
ln(A)xe2x89xa6105.4xe2x88x9245126/Bxe2x80x83xe2x80x83(1)
ln(A)xe2x89xa62545.2/Bxe2x88x9215.82xe2x80x83xe2x80x83(2)
ln(A)xe2x89xa7225.9xe2x88x92102168.8/Bxe2x80x83xe2x80x83(3)
ln(A)xe2x89xa71300.0/Bxe2x88x9215.74xe2x80x83xe2x80x83(4)
wherein A is an initiator concentration (a molar ratio of the initiator/the monomer); B is a polymerization temperature (xc2x0 K); and ln is a symbol for a natural logarithm; 
wherein R is alkyl or fluoroalkyl.
This invention also provides a process for manufacturing an optical fiber comprising preparing a (co)polymer by the above process for manufacturing a methacrylate (co)polymer comprising conducting polymerization while feeding a monomer (mixture) containing at least 90 wt % in total of at least one methacrylate monomer and a radical polymerization initiator represented by formula (II) into a reactor, which further comprises a feeding step of feeding a reaction mixture taken out from the reactor to a devolatilization step and a devolatilization step of separating and removing volatiles from the reaction mixture; and feeding the thus obtained (co)polymer and another polymer having a different refractive index to a multi-component spinning nozzle for spinning.